An implementation of a history-style drop-down menu button. The button
itself executes an action when the left side is clicked; the right side
of the button is a toggle that displays a drop-down menu of actions when
it is selected.

The two sides of the button look something like this:

+------+---+
| | |
| | _ |
| | V |
| | |
+------+---+

The drop-down menu of actions is configured via the ToggleAction.ACTION_MENU_ACTIONS property on the main button Action. The property must be set to an array of Action
objects that represent the menu choices. The action array is empty by
default.

The button itself does not alter its enabled state based on the presence
or absence of actions in the drop-down menu. If there are no actions in
the menu, the client may disable the button via the Action.setEnabled(boolean) method on the main button action, if
desired.